The invention relates to a method according to the preamble of claim 1. U.S. Pat. No. 4,559,625 to Berlekamp et al, and U.S. Pat. No. 5,299,208 to Blaum et al disclose decoding of interleaved and error protected information words, wherein an error pattern found in a first word may give a clue to locate errors in another word of the same group of words. The references use a fault model with multisymbol error bursts across plural words. An error in a particular word yields a high probability for an error to occur at a corresponding symbol position pointed at in a next word or words. The procedure may raise the number of corrected errors. A clue will however only materialize when the clue word has been fully corrected. Furthermore, the medium stores information as well as synchronization bit groups which represent an appreciable amount of redundancy that may also be used to signal errors in the target words, so that to some extent clue words may be exchanged for synchronization bit groups. Part of the errors is due to so-called bit slips that are more frequent further away from the sync bit groups. Therefore, interleaving the clue words amongst clue columns and also interleaving the target words amongst target columns may further improve the error protection.
In consequence, amongst other things, it is an object of the present invention to provide a coding format allowing clue words to cooperate with synchronizing bit groups in a systematic format, while also smoothing out possibly negative effects of bit slips more evenly among the various words. Now therefore, according to one of its aspects the invention is characterized as recited in the characterizing part of claim 1. A clue can point to an erasure symbol. Pointing can make error correction more powerful. In fact, many codes will correct at most t errors when no error locative indication is known. Given the erasure locations, generally a larger number e greater than t of erasures may be corrected. Also protection against a combination of bursts and random errors may improve. The invention may be used both for storage and for transmission.
Advantageously, the method has the features of claim 2. This is a relatively simple arrangement. Alternatively, the number of synchronizing columns may be larger than one, and the number of clue columns may be even if preferred.
Advantageously, the method has the features of claim 3. In video recording, user data will relate to the image and accompanying sound to be presented to a user, whereas system data may indicate the program name, time, addresses, and various other parameters that may be useful without reference to the video or audio proper. This feature allows fast access to the system data without necessity for decoding the target words.
Advantageously, the method has the features of claim 4. If error-free, this feature will immediately indicate to a user device whether the processed frame of information is correct.
Advantageously, the method has the features of claim 5. This is a straightforward organization. Advantageously, the method has the features of claim 6. This has proven to raise the worst case to the average level attainable for this code format. Advantageously, the method has the features of claim 7. Optical storage has been proven a fortuitous medium.
The invention also relates to a method for decoding information so encoded, to an encoding and/or decoding device for use with the method, and to a carrier containing information so encoded. Further advantageous aspects of the invention are recited in dependent claims.